The present invention relates in general to filtration modules or elements, and in particular to a new and useful method and apparatus which utilizes a curved or coiled tubular membrane constructed to maximize the formation of Dean vortices at the solution-membrane interface, for improving the filtration effect.
Most modular designs for pressure-driven membrane processes, such as reverse osmosis, ultrafiltration and microfiltration are based on maximizing membrane area per unit volume and on the handling convenience of the module. Many methods exist for reducing CP and fouling, including chemical modification of the membrane surface and physical methods such as scouring. Hydrodynamic methods are also known which rely on eddies during turbulent flow, or induced flow instabilities. Such instabilities can be created by introducing inserts into the flow path. Unstable flow across membranes have also been utilized to reduce solute build-up at the solution-membrane interface, by the inventor of the present application. See Belfort, G., xe2x80x9cFluid mechanics in membrane filtration: recent developmentsxe2x80x9d, J. Membrane Sci., 40, 123-147 (1989).
Different types of instabilities have been used including vortices and instabilities resulting from rough membrane surfaces, flow pulsations and oscillating membrane surfaces. In addition to rough membrane surface, etc., instabilities have also been induced by a rotating disc system developed (i) in the 1970""s by Fred Littman and Jerry Croopnick at Dresser Industries, TX and before that at Stanford Research Laboratories, CA and (ii) in the 1990""s by Brown Boveri Co. in Malmo, Sweden. One of the most successful depolarizing methods has used Taylor vortices established in a rotating annular filter module. The main limitations of this design are the difficulty in scaling-up membrane area and high energy consumption. Vortices have also been produced in membrane-lined channels by frequently reversing turbulent flow (at 8 Hz) in a corrugated channel. See, Stairmand, J. W. and Bellhouse, B. J., xe2x80x9cMass transfer in a pulsating turbulent flow with deposition into furrowed walls.xe2x80x9d Int. Heat Mass Transfer, 27, 1405 (1985). This has also been done by forcing the fluid to flow around in a spiral half-cylinder channel over a flat membrane. See PCT patent application WO 90/09229 of Aug. 23, 1990 to Winzeler. Both of these approaches show increased performance in the presence of vortices, but each has experienced some difficulties including scale-up and sealing problems. The use of spacers in a spiral wound unit to induce mixing has been widely recommended. Unfortunately, at low axial Reynolds numbers typically used in Poiseuille flow for most spiral wound units, this approach has been largely ineffective. See the spiral flow filters of Toray Industries, Inc., disclosed, for example, in their brochure entitled Romembra Toray Reverse Osmosis Elements.
The effective use of fluid instabilities, such as vortices, in depolarizing and cleaning synthetic membranes for pressure-driven membrane application has been widely confirmed in the literature. See Winzeler, H. B. and Belfort, G. (1993), Enhanced performance for pressure-driven membrane processes: The argument for fluid instabilities, J. Membrane Sci., in 80, 35-47. The present invention has shown excellent flux improvements in the presence of Dean vortices resulting from flow around a curved duct with microfiltration membranes. See U.S. Pat. No. 5,204,002, which is incorporated here by reference. An object of the present invention is to provide for such controlled vortices to be used to depolarize salt, macromolecules and suspensions in high pressure reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF) or nanofiltration (NF) membrane processes.
The present invention is an apparatus, and method of establishing vortices, in particular, Dean vortices, resulting from the onset of unstable flow in a curved path.
The invention includes using Dean vortices to defoul, i.e. remove deposits, and depolarize, i.e. remove suspended dissolved matter near the membrane, solutes away from membrane surfaces. The invention also includes a spiral or otherwise wound membrane tube for use in:
(i) reverse osmosisxe2x80x94mainly used for retention of salt and low molecular weight organics;
(ii) nanofiltrationxe2x80x94mainly used for fractionation of salt and low molecular weight organics;
(iii) ultrafiltrationxe2x80x94mainly used for fractionation of medium molecular weight organics and transport of salt and low molecular weight organics;
(iv) microfiltrationxe2x80x94mainly used for retention of colloids, small particles, and transport of salt, low molecular weight organics and other dissolved organics and solutes.
The Dean vortex flow of the invention not only has similar advantages as Taylor vortex and oscillating flows but also is amenable to scale-up.
To practice one example of the invention, it was necessary to experimentally determine the effective and optimum range of Dean vortex depolarization for NF; to design, build and test simple prototype vortex generating tubular NF element; to use NMR imaging methods to verify the existence and persistence of vortices; and to solve the convective-diffusion equations for fast Dean flows with variable wall flux and variable parameter properties and use this model for predictive purposes. Concentration profiles within a vortex as a function of axial distance was used to estimate the effectiveness of this approach. This research strengthens our knowledge of how to produce vortex instabilities and how to use them to diminish concentration polarization.
Accordingly, one object of the present invention is to provide a method of defouling and depolarizing a solute from a membrane surface, comprising: providing the solute in a fluid solution adjacent the membrane surface; and producing Dean vortices in the fluid solution which move across the membrane surface, to defoul and depolarize the solute away from the membrane.
A further object of the invention is to provide an apparatus for reverse osmosis (RO), microfiltration (MF), nanofiltration (NF) and ultrafiltration (UF) processes, comprising: a wound membrane tube; and means for supplying a fluid through the tube for subjecting at least one component of the fluid to reverse osmosis, nanofiltration, microfiltration or ultrafiltration, through the membrane tube.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which the preferred embodiments of the invention are illustrated.